[PLUGIN] bzr_difftools performance enhancement

[Stephen Ward]

Hi,

I've updated the bzr_difftools plugin with a significant reduction in 
the 'import' cost that would affect any user of this plugin.

With all the recent lazy_import improvements to the core, it had reached the 
point where the plugin was adding 50% to the time it takes to run 'bzr 
rocks'.  That still isn't a lot of time in any absolute sense, but it is  
depressingly large in percentage terms.

With this change, that startup overhead has been reduced to 1%.

This was done by minimizing the initial 'import' list as much as possible, and 
leaving the heavy lifting for when the command is actually run - so the cost 
is still there, but now it only affects the 'diff' command itself.

I also looked at using lazy_import within the plugin code, but that was not as 
effective in this case (it cut the overhead from 50% to 21%).

The updated code is available from the usual locations (see the Plugin 
Registry).

-- Steve